Known is a technology for retrieving a number to be retrieved, using a bitmap index representing a set of bit sequences indicating, for each file to be retrieved, whether the number is present in a group of numbers with the number of significant digits and the most significant digit matching those of the number to be retrieved (for example, see International Publication Pamphlet No. WO2008/047432).
For such a technology, an information processing apparatus is caused to detect the number of significant digits and the most significant digit of the number to be retrieved, and to extract a bit sequence indicating, for each file to be retrieved, whether the number is present in a group of numbers with the number of significant digits and the most significant digit matching those having been detected, using a bitmap index. The information processing apparatus then identifies a file to be retrieved that is mapped to a bit included in the extracted bit sequence and indicating the presence of the number, from a plurality of files to be retrieved. The information processing apparatus then refers to the identified file to be retrieved, and determines whether the number to be retrieved is present.